Power converters/inverters are commonly used in a machine for motor control. Power converters/inverters usually include a plurality of power transistors, and these power transistors may be switched on and off to modulate an output voltage from the power converter/inverter. Examples of power transistors may include bipolar junction transistors (BJT), the Darlington device, metal oxide semiconductor field effect transistors (MOSFET), and insulated gate bipolar transistors (IGBT). In particular, IGBTs have been widely used in a range of applications due to their high switching speed and ability of conducting very high current.
High voltage power converters/inverters are usually expensive, and failure of power transistor components can be costly. Power converters/inverters often fail because of thermal overload. The thermal overload is often caused by either exceeding a maximum switching frequency or exceeding a maximum current limit. A conventional power converter's/inverter's switching frequency is determined based on the rotor speed. As the rotor speed increases, so does the switching frequency, and at high rotor speeds, a thermal overload is possible. Additionally, in the event that there is a malfunction in the power transistors or the wiring, damage may be caused to other power components in the circuit. Therefore, a protection strategy is needed to prevent thermal overload, particularly from exceeding a maximum switching frequency or exceeding a maximum current.
A device and method for dynamically optimizing a power converter in an electric machine is described in U.S. Patent Publication No. 2005/0219883 to Maple et al. (“the '883 publication”). The '883 publication describes a dynamically optimized power converter unit that increases efficiency of one or more power converters supplying energy to a load. The device or method may select a starting frequency. The starting frequency may be a preprogrammed maximum allowed frequency, or may be selected from a lookup table based on of the output power. An efficiency is calculated for the selected starting frequency. The frequency is then decremented, and the efficiency is calculated and compared to the efficiency at the previously calculated frequency. When the efficiency no longer needs to be increased, the previous frequency is the optimal switching frequency. At medium power levels the switching frequency is used to control the efficiency of the power converters. At high power levels, two or more power converters may be used to share the load. The temperature is measured as the criteria for power sharing, because temperature is a good indicator of power dissipation and may be used to balance two power converters sharing a load.
Although the device and method of the '883 publication may provide a dynamically configured power converter that may increase efficiency in some cases, it may include several disadvantages. Specifically, the device and method of the '883 publication may attempt to limit thermal overload by searching for an efficient switching frequency and using more than one power converter to share the power provided to the load. The complexity and cost of the control circuit and the power converters may be increased. Because the '883 publication iteratively searches for a switching frequency, a more sophisticated processor may be required, which may increase costs. Additionally, because the device and method of the '883 publication adjust the switching frequency to increase efficiency, and at high power may use two or more power converters and balance the power sharing between the two or more power converters as a function of power converter temperature, the maximum current is not directly controlled. Thus, in order to provide increased thermal protection, a method and system may be needed that provide both thermal overload protection as a result of exceeding a maximum switching frequency and/or exceeding a maximum current.
The disclosed method and system are directed to improvements in the existing technology.